methods to relate estimates for different time periods, such as using 1-year period estimates for a state, large county, or PUMA to update 5-year period estimates for small governmental jurisdictions (see Section 3-B.3).

In using the ACS data to study trends and changes over time, users will need to keep in mind the implications of changes in an area’s geographic boundaries and population size for their analysis. With regard to population size, a governmental unit may gain or lose population so that it crosses a population size threshold for the publication of estimates. For example, a small city may grow from 60,000 to 70,000 over a 5-year period. Beginning in the year when the city achieves 65,000, it will have 1-year as well as 3-year and 5-year period estimates produced. Significant population decline, however, if sustained, could cause an area to be dropped from the 1-year or even the 3-year period estimates series. Population changes may also increase or decrease the initial sampling rate for an area.

With regard to boundaries, the Census Bureau will continue to update regularly the geographic boundaries of most types of governmental units every year—for example, to reflect an annexation or a combination or splitting up of units. It will update school district boundaries every 2 years and update the boundaries of statistical areas, including metropolitan areas, urbanized areas, PUMAs, census tracts, and block groups every decade in conjunction with the census. For ACS estimates for such governmental units as counties and cities, the Census Bureau will use the geographic boundaries as of January 1 of the most recent year of data that figure into the particular set of estimates. Consider a large city that annexed territory in late 2008 for which the user is working with 1-year, 3-year, and 5-year period estimates pertaining to 2010, 2008–2010, and 2006–2010, respectively. All these estimates will include data for the current enlarged city boundaries. The Census Bureau will not, however, revise estimates that precede the most recent 5 years to reflect boundary changes.

2-C.2
Sampling Error

The use of a sample rather than a complete enumeration introduces sampling error that affects the precision of the estimates from a survey. Such error is related to the variability of the characteristic in the population, the size of the sample, and the sample design. For a given estimate and sample design, the larger the sample size, the smaller is the sampling error.

2-C.2.a
Design Factors

Overall, the ACS 5-year period estimates for an area will exhibit greater sampling error than the 2000 census long-form-sample estimates for the same area. (The sampling errors for 3-year and 1-year period estimates will be greater yet.) Two reasons are that the ACS cumulative 5-year initial



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